1. Field of the Invention
This invention is directed to an improved process for vapor-phase disproportionation of toluene in the presence of catalyst comprising a crystalline molecular sieve characterized by a silica/alumina mole ratio of at least about 12 and a Constraint Index of from about 1 to about 12. The improvement comprises use of a molecular sieve catalyst component having a high lattice aluminum content whereby its silica/alumina mole ratio is less than 55 and a diffusion rate constant of less than 150 sec.sup.-1.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline molecular sieves having a definite crystalline structure within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Prior art techniques have resulted in the formation of a great variety of synthetic molecular sieves. These materials have come to be designated by convenient symbols, as illustrated by ZSM-5 (U.S. Pat. No. 3,702,886).
The use of certain molecular sieves as catalyst components is taught in U.S. Pat. No. 4,305,808, for example.
The silica/alumina molar ratio of a given molecular sieve is often variable; for example, zeolite X (U.S. Pat. No. 2,882,244) can be synthesized with a silica/alumina ratio of from 2 to 3; zeolite Y (U.S. Pat. No. 3,130,007) from 3 to about 6. In some molecular sieves, the upper limit of silica/alumina ratio is virtually unbounded. ZSM-5 is one such material wherein the silica/alumina ratio is at least 5. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina and exhibiting an X-ray diffraction pattern characteristic of ZSM-5. U.S. Pats. Nos. 4,061,724; 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
It is known that zeolites are stabilized for various processes by reducing lattice aluminum content. The FCC applications, for example, the catalyst of choice is ultrastable Y which has been dealuminized from its precursor Y form by steaming. Another example of stability enhancement by catalyst dealuminization is in hydrodewaxing. U.S. Pat. No. 4,247,388 teaches the improvement of catalyst aging characteristics in lube dewaxing by steaming ZSM-5 to reduce lattice aluminum content.
U.S. Pat. No. 4,380,685 teaches para-selective alkylation, transalkylation or disproportionation of a substituted aromatic compound to form a dialkylbenzene compound mixture over catalyst comprising zeolite characterized by a Constraint Index of 1 to 12 and a silica/alumina mole ratio of at least 12/1, the catalyst having thereon incorporated various metals and phosphorus. Other patents covering alkylation and transalkylation include U.S. Pats. Nos. 4,127,616; 4,361,713; 4,365,104; 4,367,359; 4,370,508 and 4,384,155. Toluene is converted to para-xylene in U.S. Pats. Nos. 3,965,207; 3,965,208; 3,965,209; 4,001,346; 4,002,698; 4,067,920; 4,100,215 and 4,152,364, to name a few. Alklyation with olefins is taught, for example, in U.S. Pats. Nos. 3,962,364 and 4,106,218 and toluene is disproportionated in, for example, U.S. Pats. Nos. 4,052,476; 4,007,231; 4,011,276; 4,016,219 and 4,029,716. Isomerization of xylenes is taught in, for example, U.S. Pats. Nos. 4,100,214; 4,101,595; 4,158,676; 4,159,282; 4,351,979; 4,101,597; 4,159,283; 4,152,363; 4,163,028; 4,188,282 and 4,224,141.
U.S. Pats. Nos. 3,551,509 and Re. 27,639 disclose transalkylation between trimethylbenzenes and toluene to yield xylenes and benzene in the presence of a crystalline aluminosilicate catalyst having large pore openings of 8 to 15 Angstrom units and, preferably containing Group VIII metals, hydrogen and rare earth cations.
In the area of aromatic disproportionation, Grandio et al teach in the Oil and Gas Journal, Vol. 69, Number 48 (1971) a liquid-phase toluene disproportionation process utilizing zeolite catalysts in the absence of hydrogen. They further teach that vapor-phase toluene disproportionation requires hydrogen recycle or else frequent regeneration of catalyst to keep coke levels low on the catalyst and to maintain catalytic activity over any reasonable period of time.
Otani teaches in Chemical Engineering, 77 (16), 118 (1970) that vapor-phase catalytic disproportionation of toluene requires hydrogen recycle to keep the zeolite catalyst from excessive coke build-up and, thereby, maintain reasonable catalyst activity.
U.S. Pats. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalyst. U.S. Pat. No. 4,117,026, incorporated herein in its entirety by reference, teaches disproportionation over catalyst comprising zeolite having a silica/alumina mole ration of at least 12, a Constraint Index of 1 to 12 and a specified sorption capacity for xylenes.